Here is a vivid real world illusion, spotted and photographed by my friend Eddy Keon: it’s a brand new wall in Nottingham, UK, and it’s perfectly vertical, but with these shadows, it looks twisted.

The twist seems to be due just to the way the edges of the shadows running down the wall lean over more and more as the eye travels from right to left across the photo. It’s a kind of crazy perspective effect.

When I first saw the photo, I thought the twist was enhanced by the Zöllner illusion, which I’ve posted about before. If that was so, the long shadow edges on the wall would appear to lean over even more, thanks to an effect from the horizontal masonry lines that run across them. But as shown below, the masonry lines, added to the left, don’t seem to affect the orientation of the shadow lines at all. I’m not sure why not.

The movie in the middle is a clip from the one shown (larger) in the previous post. The illusion is that the objectively static sloping sides of the V shape seem to move sideways horizontally. All around it are some of the ways of reducing or cancelling the illusion, hopefully offering some clues about what is causing it (which for now is a mystery). Top left is an empty triangle against the clouds – no illusion. Centre left is the movie with no sky background – slightly reduced illusion, in our judgment. Lower left the clouds are not moving – no illusion, we reckon. Then top right, clouds moving in the opposite direction to the figure in the movie in the V-shape – no illusion; centre right, no squeeze and stretch distortion of the movie in the V-shape – no illusion; & bottom right, a more complicated one: the figure in the V shape accelerates as he moves upwards, getting bigger and moving faster as he rises, so that the figure seems to loom out towards us as he rise. The expansion illusion is replaced with an impression of depth.

We’ll post again on this illusion when we (or someone else) puzzles out what’s going on. But don’t hold your breath ….

We just got into the final top ten entries in the annual Best Visual Illusion of the Year contest. We didn’t get in the top three, (in fact, we seem to have come last … ) but it’s great to have made the top ten. The movie here is a different version of our competition entry.

The illusion is that the objectively static sides of a V-shaped window appear either to expand or to contract horizontally. Figures within the window, expanded at the top and squashed at the bottom into the V-shape, rise or fall at constant speed. In the middle section of the movie, with coloured figures, you can see that the moving bacground around the V-shapes is not essential, the illusion is still there without it, maybe reduced. You can also see that with figures that just rise, without the squeeze and squash, there’s no illusion.

At the end of this version, we show that we’ve actually found three fairly different ways of producing this illusion. We found them by studying the reflections seen in novelty illusion rings called Witch Rings as they rotate. We posted about using animations to imitate effects seen in the reflections, in 2013, and then in 2015 and 2016. But now we think the illusion we’ve found in our animations, though an interesting discovery, only makes a small contribution to the very vivid illusion seen in the rings in the real world – the secret of the rings remains mysterious!

I’ll post with some more demonstrations of what we think may be going on in the illusion in a few days.

I’ve copied this beautiful demo (with small changes) from one by Stuart Anstis, who is one of the world’s leading and most prolific researchers into illusions. His website includes a page of great movies, including this one. Whilst the yellow circles are visible, we tend to focus locally on the pairs of spheres, each pair orbiting a central point. But without the circles, loosely fixate the central blob, and though the movement of the spheres remains just the same, they appear to re-group into a more global view, of two pulsating, intersecting circles of spheres.

I came across Stuart’s movie amongst the many web pages of figures and demonstrations that accompany a once-in-a-generation, landmark publication, the Oxford Compendium of Visual Illusions. (It’s not cheap – check the price before ordering!). But that’s because it’s HUGE, with some 800 pages. Almost all the leading researchers in the field worldwide have contributed, with essays on the history of visual illusions, up-to-the-minute, detailed discussions of a comprehensive range of illusions and effects, and philosophical essays on whether the word illusion is really the right term to describe them.

This may not look a dramatic illusion by contemporary standards: the top and bottom lines are each divided into three equal segments, but the middle segment appears longer than the flanking segments in the top line, and vice versa below. Yet it’s a really remarkable figure. It’s one of sixteen in a pioneering paper of 1855, the first ever study of illusions of this geometric kind. Even more remarkably, as an illusion it remains to this day wholly unexplained, as baffling as it is simple.

The paper was by a Frankfurt schoolmaster called Johann Joseph Oppel. In it he named his illusions as geometric-optical illusions, to distinguish them from illusions observed in the natural environment such as the Moon illusion. Oppel seems to have observed the misjudgments that these new geometric illusions give rise to in math lessons, when his pupils were drawing or judging figures on the blackboard. I suspect he was probably an unforgettable but demanding teacher. He was for sure a real one-off – a tirelessly curious observer, leading the way into acoustical and language research, as well as geometric illusions, with fearless independence.

In the era of Trump it’s got harder to tell reality from illusion, but here’s a ceiling from about 300 hundred years ago that shows a real magician in the illusion line at work. The inside of the dome – everything inside the gold circle – looks as if it extends upwards, but it is just painting on a flat canvas. It’s an example of so-called trompe l’oeil, French for deceive the eye. I don’t know why we use a French term – it was never a French speciality.

Some time back I posted about the brilliant trompe-l’oeil by Andrea Pozzo on the ceiling of a church in Vienna, painted a little over three hundred years ago. The photos here show a ceiling in the UK painted thirty years later, by an artist following where Pozzo led the way. It’s in the main entrance hall of a mansion called Moor Park, in the West London suburb of Ruislip, painted in the 1730s. The mansion is now a posh golf club, so it’s not usually open to non-members, but it’s visitable by arrangement, http://www.moorparkgc.co.uk/

The painting of the hallway was done by a trio of Italian specialists in architectural decoration – Giacomo Amiconi, Gaetano Brunetti and Francesco Sleter. I’m not sure whether Brunetti or Sleter painted the illusory dome. They’re not well known. Painting this kind of thing was quite lucrative, but didn’t make you a star in the art world of the time.

Like the dome painted by Pozzo, the illusion in Moor Park is amazingly successful – but only from one viewpoint. As you can see in the lower photo, from everywhere else the dome looks wonky.

This is a post to show animations to accompany a poster, which I and my colleague Priscilla Heard presented at ECVP 2016. The poster reported experiments related to the witch ring illusion. The movies below show illusory effects of sideways movement in streaming patterns of dots.

This first movie shows how the static track along which a single file of dots are travelling appears to move sideways when the single track is embedded in a fan-shaped pattern of tracks.

The eyes in each pair of pictures don’t change at all, and yet in one picture in each pair they seem to look directly at us, whilst in the other they have rotated downwards. (featured: a Sportive Lemur; & a young man by German 16th century sculptor Michel Erhart in the Victoria and Albert Museum, London).

William Wollaston published this illusion nearly 200 years ago. He claimed that our brains seem set up to judge the direction in which the eyes are looking in relation to the face from the position of the pupils in relation to the whites of the eyes, but that direction of gaze depends on head orientation. You can read his original paper in the Philosophical Transactions of the Royal Society online. But here’s an animation of the original illustrations:

The original drawings were done for Wollaston by the leading portrait painter of his day, Sir Thomas Lawrence. They are now in the Royal Society in London, and there’s a movie about them you can watch on Youtube.

Here’s another version of the illusion I’ve been working on with my colleague Priscilla Heard. The bright V shaped zones imposed on the faces look like they are expanding or contracting sideways, especially near the sharp end of the Vs at the bottom. It can be hard to see that it’s an illusion, because the faces really are expanding. But to the right an isolated bright zone shows that objectively the V tracks are quite static. You may still see a bit of illusory lateral expansion even in this isolated set of tracks, but note that the top right corner of the track is quite static in relation to the corner of the movie.